Surface-emitting semiconductor light device

ABSTRACT

A semiconductor light emitting device is disclosed in which a semiconductor multilayer structure including a light emitting layer is formed on a substrate and light is output from the opposite surface of the semiconductor multilayer structure from the substrate. The light output surface is formed with a large number of protrusions in the form of cones or pyramids. To increase the light output efficiency, the angle between the side of each protrusion and the light output surface is set to between 30 and 70 degrees.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Applications No. 2001-191724, filed Jun. 25,2001; and No. 2001-297042, filed Sep. 27, 2001, the entire contents ofboth of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor light emitting diode,such as a light emitting diode (LED) or a laser diode (LD). Morespecifically, the present invention relates to a semiconductor lightemitting device having its light output surface made rough.

2. Description of the Related Art

Conventionally, a high intensity LED has been fabricated by forming adouble-heterostructure light emitting region on a semiconductorsubstrate and then forming a current diffusing layer on the lightemitting region. For this reason, packaging the high intensity LED witha resin results in a structure in which the top of the current diffusinglayer is covered with the passivating transparent resin.

With such a structure, the critical angle associated with the currentdiffusing layer (the refractive index is in the range of 3.1 to 3.5) andthe vitreous resin (the refractive index is of the order of 1.5) is inthe range of 25 to 29 degrees. Of light that travels from the lightemitting region toward the vitreous resin, the light that strikes thelayer—resin interface at angles larger than the critical angle willsuffer total internal reflections. This will significantly reduce theprobability of light produced within the LED being emitted to theoutside. At present, the probability (light output efficiency) is of theorder of 20%.

There is a method of improving the light output efficiency by forming afilm of high refractive index on the current diffusing layer to therebyincrease the critical angle. However, even with this method, an increasein the efficiency is low, of the order of 20%.

Thus, the conventional LEDs that are packaged with transparent resinmaterial suffer from the problem that the light output efficiency islow. This is because, at the interface between the transparent resin andthe top layer of semiconductor multi-layer structure including a lightemitting layer, most of the light that strikes the interface at anglessuffers total internal reflections. This problem is common tosurface-emitting LDs.

BRIEF SUMMARY OF THE INVENTION

According to an aspect of the present invention, there is provided asurface-emitting semiconductor light emitting device comprising; asubstrate having a major surface; a semiconductor multilayer structureformed on the major surface of the substrate and including a lightemitting layer, emitted light being output from the opposite surface ofthe multilayer structure from the substrate; and a plurality ofprotrusions formed on the light output surface of the semiconductormultilayer structure, the angle between the base and side of eachprotrusion being set to between 30 and 70 degrees.

According to another aspect of the present invention, there is provideda surface-emitting semiconductor light emitting device comprising: asubstrate having a major surface; and a semiconductor multilayerstructure formed on the major surface of the substrate and including alight emitting layer, light being output from the opposite surface ofthe semiconductor multilayer structure and the light output surfacehaving been subjected to a roughening process so that a large number ofprotrusions and recesses is formed thereon, the distance between thepeak and valley of each protrusion and recess being set to between 50 nmand the wavelength of light emitted by the light emitting layer.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIGS. 1A, 1B and 1C are cross-sectional views, in the order of steps ofmanufacture, of a green LED according to a first embodiment of thepresent invention;

FIG. 2 is an enlarged cross-sectional view of protrusions formed on thelight output surface of the LED of FIG. 1;

FIG. 3 is a plan view of an electrode pattern of the LED of FIG. 1;

FIG. 4 is a plot of the angle between the side of the protrusion and thesubstrate surface versus the light output efficiency of the LED of FIG.1;

FIG. 5 is a cross-sectional view of a green LED according to a secondembodiment of the present invention;

FIGS. 6A and 6B are cross-sectional views, in the order of steps ofmanufacture, of a green LED according to a third embodiment of thepresent invention;

FIG. 7 is an enlarged cross-sectional view of the neighborhood of thelight output surface in the third embodiment;

FIG. 8 is a cross-sectional view of a surface-emitting LD according to athird embodiment of the present invention;

FIGS. 9A, 9B and 9C are cross-sectional views, in the order of steps ofmanufacture, of a green LED according to a fifth embodiment of thepresent invention;

FIG. 10 shows a plot of the light output efficiency versus the height ofprotrusions in the LED of FIG. 5;

FIG. 11 shows a plot of the light output efficiency versus the height ofprotrusions comparable in size with the emitted wavelength;

FIG. 12 shows a plot of the light output efficiency versus therefractive index when the surface of the antireflection film isroughened;

FIG. 13 shows a plot of the light output efficiency versus therefractive index when the surface of the antireflection film is madesmooth;

FIGS. 14A to 14E are sectional views illustrating various surfaceconfigurations of the antireflection film which may be used in theinvention;

FIG. 15 is a cross-sectional view of a green LED according to a sixthembodiment of the present invention; and

FIG. 16 is a cross-sectional view of a surface-emitting LD according toa seventh embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The preferred embodiments of the present invention will be describedwith reference to the accompanying drawings.

FIGS. 1A, 1B and 1C are cross-sectional views, in the order of steps ofmanufacture, of a green LED according to a first embodiment of thepresent invention.

First, as shown in FIG. 1A, onto an n-type GaAs substrate 10 of 250 μmin thickness, an n-type GaAs buffer layer 11 of 0.5 μm in thickness isgrown by means of metal-organic CVD (MOCVD) using AsH₃ as a group Velement source gas. After that, by means of MOCVD using PH₃ as a group Velement source gas and under conditions of a PH₃ partial pressure of 200Pa and a total pressure of 5×10³ Pa, an n-type In_(0.5)Al_(0.5)Pcladding layer 12 of 0.6 μm in thickness and an undopedIn_(0.5)(Ga_(0.55)Al_(0.45))_(0.5)P active layer 13 of 1.0 μm inthickness are grown in sequence.

Subsequently, a p-type In_(0.5)Al_(0.5)P cladding layer 14 of 1.0 μm inthickness is grown by means of MOCVD with the PH₃ partial pressurereduced to 10 Pa and the total pressure kept at 5×10³ Pa. After that, ap-type GaAs contact layer 16 of 0.1 μm in thickness is grown by means ofMOCVD using AsH₃ as a group V element source gas. Each of the epitaxiallayers from the buffer layer 11 to the contact layer 16 is grown insuccession in the same chamber.

As described above, in growing the p-type InAlP cladding layer 14, whenthe PH₃ partial pressure in the MOCVD process is reduced to asufficiently low pressure (not higher than 20 Pa), the surface of theepitaxial layer becomes roughened. To be specific, conical protrusions20 are produced on the surface of the InAlP cladding layer 14 as shownin FIG. 2. The angle of each protrusion with respect to the substratesurface, i.e., the angle α made by the base and the side of eachprotrusion, becomes larger than 30 degrees.

Here, if, when the InAlP cladding layer 14 is being grown, the PH₃partial pressure is in excess of 20 Pa, the surface of the claddinglayer would become less roughened, increasing the possibility that eachprotrusion may fail to attain more than 30 degrees in the angle α madeby its base and side. If, on the other hand, the PH₃ partial pressure islower than 1 Pa, then the surface of the cladding layer 14 would becometoo much roughened and moreover its crystallinity would also becomedegraded. Therefore, the PH₃ partial pressure at the growth of the InAlPcladding layer 14 should preferably be in the range of 1 to 20 Pa.

Next, as shown in FIG. 1B, an ITO (Indium Thin Oxide) film 17 serving asa transparent electrode is formed on a selected portion of the GaAscontact layer 16 through sputtering techniques. Subsequently, a p-sideelectrode (Zn-containing Au) 23 is formed on the ITO film 17. Morespecifically, a current block layer 21 and a GaAs layer 22 are grown onthe ITO film 17 and then selectively etched so that they are left in thecentral area of the chip. Subsequent to this process, the AuZn electrode23 is formed over the entire surface and then patterned so that it isleft on the GaAs film 22 and selected portions of the ITO film 17.

FIG. 3 is a plan view illustrating a pattern of the p-side electrode 23.This electrode pattern is comprised of a circular pad 23 a provided inthe central area of the device so that a wire may be bonded, peripheralportions 23 provided along the edges of the device, and contact portions23 c that connect the peripheral portions 23 b to the central pad 23 a.

Next, as shown in FIG. 1C, the GaAs substrate 10 has its rear sidepolished to a thickness as small as 100 μm and then formed underneathwith an n-side electrode (Ge-containing Au) 25. After that, theresulting structure is subjected to a heat treatment in an Ar atmosphereat 450° C. for 15 minutes. Subsequently, the substrate 10, formed withthe layers 11, 12, 13, 14, 16, 17, 21 and 22 and the electrodes 23 and25, is scribed to obtain chips. Each individual chip is then housed in aresin package so that its light output surface is covered with atransparent resin not shown.

A single chip structure is illustrated in FIG. 1; however, in practice aplurality of chip structures as shown in FIG. 1 is formed on the samesubstrate 10 in order to form a plurality of chips at the same time. Byscribing the substrate 10 at the final stage, it is separated intochips.

According to the present embodiment, as described above, the conicalprotrusions 20 can be formed on the surface of the cladding layer 14 bysetting the PH₃ partial pressure lower than usual when the p-type InAlPcladding layer 14 is grown. The formation of the protrusions 20 allowsthe probability of incident light suffering total internal reflectionsat the interface between the topmost layer of the multi-layer structureincluding the light emitting layer and the transparent resin to bereduced. In particular, by setting the InAlP cladding layer growth timePH₃ partial pressure to between 1 and 20 Pa, the angle a made by thebase and the side of each protrusion can be set to 30 degrees or more.

Here, a relationship between the incidence-on-resin probability (lightoutput efficiency) and the angle of the protrusions 20 with respect tothe substrate surface is shown in FIG. 4. In this figure, the angle isshown on the horizontal axis and the light output efficiency is shown onthe vertical axis. The light output efficiency when the surface is freeof protrusions and hence flat is taken to be unity. An improvement ofmore than ten percent was recognized when the angle α was 30 degrees ormore. Conversely, when the angle α was too large, a reduction in theefficiency was recognized. With angles in excess of 70 degrees, theefficiency fell below ten percent. Thus, the angle α should preferablyrange from 30 to 70 degrees.

The adoption of the protrusion structure as in this embodiment allowsthe light output efficiency to be increased by a factor of 1.15 incomparison with the conventional device without protrusions. That thelight output efficiency can be increased without changing the basicdevice structure is extremely advantageous to LEDs.

Even though the angle α should be set to 30 degrees or more, all theprotrusions need not necessarily meet this requirement. Most of theprotrusions (for example, more than 90 percent) have only to meet therequirement. Even if each protrusion is formed so as to have an angle αin the range of 30 to 70 degrees, some of the resulting protrusions mayhave an angle of less than 30 degrees and some of them may have an angleof more than 70 degrees. There will arise no problem if the percentageof protrusions that have angles outside the range of 30 to 70 degrees issufficiently low.

Thus, according to the present embodiment, the light output efficiencycan be significantly improved by setting the angle α made by the baseand the side of each protrusion to between 30 and 70 degrees, not bysimply making the light output surface rough.

When the pitch or period of the protrusions 20 formed on the lightoutput side is made extremely small, the effect of increasing the lightoutput efficiency is reduced. According to our experiments, satisfactoryeffects were confirmed when the pitch of the protrusions was 0.5 μm ormore. The current block layer 21 and the GaAs layer 22 on thetransparent electrode 17 are not necessarily required. Even when themetal electrode 23 was directly formed on the transparent electrode 17,we confirmed similar effects.

[Second Embodiment]

Referring now to FIG. 5, there is illustrated, in sectional view, thestructure of a green LED according to a second embodiment of the presentinvention.

In the second embodiment, each of grown layers is opposite inconductivity type to a corresponding one of the grown layers in thefirst embodiment and the basic structure and the method of manufactureof the LED remain unchanged from those of the first embodiment.

Onto a p-type GaAs substrate 30 are sequentially grown by means of MOCVDa p-type GaAs buffer layer 31 of 0.5 μm in thickness, a p-typeIn_(0.5)Al_(0.5)P cladding layer 32 of 0.6 μm in thickness, an undopedInGaAlP active layer 33 of 1.0 μm in thickness, an n-typeIn_(0.5)Al_(0.5)P cladding layer 34 of 1.0 μm in thickness, and ann-type GaAs contact layer 36 of 0.1 μm in thickness. A transparent ITOfilm 37 is then formed on the contact layer 36 by means of sputteringtechniques.

As in the first embodiment, in growing the n-type InAlP cladding layer34, the PH₃ partial pressure in the MOCVD process is reduced to asufficiently low pressure (20 Pa or below). Thereby, conical protrusionsare produced on the surface of the InAlP cladding layer 34 as in thefirst embodiment. The angle α of each protrusion with respect to thesubstrate surface becomes 30 degrees or more.

A current block layer 41 and a GaAs layer 42 are formed on a selectedportion of the ITO film 37 and an n-side electrode 43 consisting of AuGeis formed on selected portions of the GaAs layer 42 and the ITO film 37.The GaAs substrate 30 is formed underneath with a p-type electrode 45made of ZnAu.

With such a structure as described above, the conical protrusions formedon the surface of the n-type InAlP cladding layer 34 allows theprobability of incidence of light on the transparent resin for packagingto be increased as in the first embodiment.

[Third Embodiment]

FIGS. 6A and 6B are sectional views, in the order of steps ofmanufacture, of a green LED according to a third embodiment of thepresent invention.

First, as shown in FIG. 6A, onto an n-type GaAs substrate 50 of 250 μmin thickness are sequentially grown by means of MOCVD an n-typeIn_(0.5)Al_(0.5)P cladding layer 12 of 0.6 μm in thickness, an undopedIn_(0.5)(Ga_(0.55)Al_(0.45))_(0.5)P active layer 53 of 1.0 μm inthickness, a p-type In_(0.5)Al_(0.5)P cladding layer 54 of 1.0 μm inthickness, an n-type InGaP current diffusing layer 55 of 3.0 μm inthickness, and a p-type GaAs contact layer 56 of 0.1 μm in thickness.For epitaxial growth of these layers, the MOCVD techniques are used asin the first embodiment.

Next, an annealing step is performed at a temperature equal to or higherthan the epitaxial temperature (not lower than 600° C.) in order tochange the epitaxial surface topography. Thereby, the surface of thecurrent diffusing layer 55 becomes roughened to form protrusions.Afterward, a p-side electrode 63 is formed on the current diffusinglayer 55 and an n-side electrode 65 is formed on the back of thesubstrate 50. Subsequently, the exposed portion of the p-GaAs layer 56is removed. Thus, the structure shown in FIG. 6B is completed.

Here, the surface roughening by annealing will be described in moredetail. As the gases used in the annealing step, an inert gas, such ashydrogen, and a group V element gas (for example, AsH₃) different fromgroup V elements (for example, p) constituting the epitaxial films(III-V compound materials, for example, InGaAlP) are introduced. Thegroup V element (P) in the epitaxial surface layer is reevaporated.Further, as the next step, an epitaxial step is performed on theroughened surface (the type of film: a transparent film of, say, GaP).

Thus, P is extracted from the surface of the InGaP current diffusinglayer 55, so that the surface becomes roughened as shown in FIG. 7. Atransparent GaP layer 57 is then grown on the rough surface of the InGaPlayer 56. The desired surface topography for increasing the light outputefficiency is one in which a large number of convex conical protrusionsare formed over the surface, which is obtained from a conventionalepitaxial surface in a state of mirror surface (Rmax=5 nm). The angle ofeach conical protrusion with respect to the base is larger than 30degrees.

With such a structure as well, the conical protrusions formed on thesurface of the current diffusing layer 55 allows the probability ofincidence of light on the transparent resin for packaging to beincreased as in the first embodiment.

The p-type GaAs contact layer 56 need not necessarily be removed;however, if it absorbs light of the emitted wavelength, it shouldpreferably be removed.

[Fourth Embodiment]

In FIG. 8, there is illustrated, in sectional view, the structure of asurface-emitting LD according to a fourth embodiment of the presentinvention.

First, on an n-type GaAs substrate 70 of 250 μm in thickness aresequentially grown an n-type GaAs buffer layer 71 of 0.5 μm in thicknessand a DBR reflecting layer 78 consisting of stackedn-In_(0.5)Al_(0.5)P/n-GaAs films.

Subsequently, an n-type In_(0.5)Al_(0.5)P cladding layer 72 of 0.6 μm inthickness, an undoped MQW active layer 73 ofIn_(0.5)(Ga_(0.55)Al_(0.45))_(0.5)P/In_(0.5)Ga_(0.5)P, and a p-typeIn_(0.5)Al_(0.5)P cladding layer 74 of 0.6 μm in thickness are grown insequence, thus forming a double heterostructure. Subsequently, a DBRreflecting layer 79 consisting of stacked p-In_(0.5)Al_(0.5)P/p-GaAsfilms, a p-type In_(0.5)Al_(0.5)P current diffusing layer 76 of 1.0 μmin thickness and a p-type GaAs contact layer 77 of 0.1 μm in thicknessare grown in sequence.

Each of the epitaxial layers from the buffer layer 71 to the contactlayer 77 is grown in succession in the same chamber through the use ofMOCVD techniques. The type of gas used and the pressure thereof areselected so that each layer is grown well. In growing the currentdiffusing layer 76, as in the first embodiment, the PH₃ partial pressureis reduced to a sufficiently low value of, for example, 10 Pa so as toallow the layer surface to become roughened.

Next, a resist pattern is formed on the contact layer 77 and the layersthrough the n-type cladding layer 72 are then etched away using theresist pattern as a mask to thereby form a laser ridge. Subsequently, aninsulating film 81 is formed except the top of the ridge and then ap-type electrode (Zn-containing Au) is evaporated. Using a resist mask,a portion of the p-type electrode which is located in the centralportion of the ridge and the p-GaAs contact layer 77 underlying thatportion of the p-type electrode are removed, thereby forming an upperelectrode 83. Subsequently, the GaAs substrate 70, after having its rearside polished to a thickness of 100 μm, is formed underneath with ann-side electrode (Ge-containing Au) 85. Next, a heat treatment iscarried out in an Ar atmosphere at 450° C. for 15 minutes. Subsequently,the substrate 70 is scribed to obtain chips. Each individual chip isthen housed in a resin package.

According to the fourth embodiment thus configured, reducing the PH₃partial pressure at the growth of the p-type current diffusing layer 76allows protrusions (irregularities) to be formed on the surface of thatcurrent diffusing layer and the angle between the surface of theresulting conical protrusions and the base to be made larger than 30degrees. In the fourth embodiment, as in the first embodiment, the lightoutput efficiency can therefore be increased. Although the laser diodeof the fourth embodiment is adapted to emit red light, this is notrestrictive. We confirmed the above effects for other laser diodes thanred diodes.

The p-type GaAs contact layer 77 need not necessarily be removed;however, if it absorbs light of the emitted wavelength, it shouldpreferably be removed.

[Fifth Embodiment]

FIGS. 9A, 9B and 9C are sectional views, in the order of steps ofmanufacture, of a green LED according to a fifth embodiment of thepresent invention.

First, as shown in FIG. 9A, onto an n-type GaAs substrate 110 of 250 μmin thickness, an n-type GaAs buffer layer 11 of 0.5 μm in thickness isgrown by means of MOCVD using AsH₃ as a group V element source gas.After that, by means of MOCVD using PH₃ as aV group element source gasand under conditions of a PH₃ partial pressure of 200 Pa and a totalpressure of 5×10³ Pa, an n-type In_(0.5)Al_(0.5)P cladding layer 112 of0.6 μm in thickness, an undoped InGaAlP active layer 113 of 1.0 μm inthickness, a p-type In_(0.5)Al_(0.5)P cladding layer 114 of 1.0 μm inthickness and a p-type InGaP current diffusing layer 115 of 1.0 μm inthickness are grown in sequence. Subsequently, a p-type GaAs contactlayer 116 of 0.1 μm in thickness is grown by means of MOCVD using AsH₃as a group V element source gas. Each of the epitaxial layers from thebuffer layer 111 to the contact layer 116 is grown in succession in thesame chamber.

Next, as shown in FIG. 9B, an antireflection film 117 is formed, whichfeatures this embodiment. That is, the antireflection film 117 having arefractive index of 2.0 and prepared by adding TiO₂ to a polyimide resinis formed on the contact layer 116 by spin coating and its surface isthen press shaped by a metal mold having protrusions which are less insize than the wavelength of emitted light. Thereby, the surfaceroughness (PV value (max−min)) of the antireflection film 117 is set tobe less than the wavelength of emitted light. Here, the PV value refersto the distance (height) between the peak and the valley of eachprotrusion.

Next, the antireflection film 117 is formed on top with a resist mask(not shown) and then removed by RIE in the place where an upperelectrode is to be formed. The resist mask is then removed.Subsequently, as shown in FIG. 9C, an electrode material (Zn-containingAu) is evaporated onto the antireflection film 117 and the exposedcontact layer 116 and then patterned using a resist mask (not shown),thus forming the upper electrode (p-side electrode) 118. The pattern ofthe p-side electrode 118 remains unchanged from that shown in FIG. 3.

Next, the GaAs substrate 110 has its rear side polished to a thicknessof 100 μm and then formed underneath with a lower electrode 119(Ge-containing Au) serving as the n-side electrode. After that, theresulting structure is subjected to a heat treatment in an Ar atmosphereat 450° C. for 15 minutes. Subsequently, the substrate 110 is scribed toobtain chips. Each individual chip, after wire bonding, is encapsulatedwith epoxy-based resin (n is about 1.5).

Thus, according to the sixth embodiment, by causing the surface of theantireflection film 117 to become roughened, the light output efficiencywas increased from about 20% (the value of the conventional device) toabout 30%. That is, the light output efficiency was increased by afactor of 1.15 in comparison with the conventional device. That thelight output efficiency can be increased by such an amount withoutchanging the basic device structure is extremely advantageous to LEDs.

FIG. 10 shows the relationship between the PV value and the light outputefficiency. As the PV value increases, the light output efficiencyincreases. When the PV value exceeds 50 nm, the light output efficiencybecomes 1.5 or more. When the PV value exceeds 200 nm, the light outputefficiency becomes about 2 and remains almost constant. FIG. 11 showsthe relationship between the PV values corresponding to wavelengthsincluding the wavelength of emitted light and the light outputefficiency. At PV values corresponding to wavelengths shorter than 640nm, the emitted wavelength, a sufficient light output efficiency isobtained. However, when the PV value increases above the valuecorresponding to the emitted wavelength, the light output efficiencydecreases sharply. Therefore, the PV value should preferably be rangedfrom 200 nm to less than a value corresponding to the emittedwavelength.

Note that all the protrusions and recesses need not necessarily meet therequirement that the PV value be ranged from 200 nm to a valuecorresponding to the emitted wavelength and most of them (for example,not less than 90%) have only to meet the requirement. That is, even ifthe protrusions and recesses are formed so as to satisfy the requirementthat 200 nm≦PV≦emitted wavelength, some of them may be outside therange. If the percentage of such protrusions and recesses is low enough,no problem will arise.

FIG. 12 shows the relationship between the light output efficiency andthe refractive index when the surface of the antireflection film is maderough. This indicates the percentage of light that is output from asurface of the antireflection film when light falls on the other surfaceof that film at an angle of incidence of −90 to +90 degrees. From FIG.12 it can be seen that, when reference is made to the light outputefficiency at a refractive index of 1.5 (the same as that of theunderlying semiconductor layer), the light output efficiency isincreased by about 50% at a refractive index of 2.0 (this embodiment)and by about 100% at a refractive index of 2.5.

FIG. 13 shows the relationship between the light output efficiency andthe refractive index when the surface of the antireflection film issmoothed. In this case, the light output efficiency is increased by 8%at a refractive index of 2.0. Even at a refractive index of 2.5, anincrease in the light output efficiency is as low as 9%. From this itcan bee seen that, in order to increase the light output efficiency, itis necessary not only to increase the refractive index of theantireflection film but also to make its surface rough.

Our experiments confirmed that the light output efficiency was increasedsufficiently by setting the surface roughness (PV value (max−min)) ofthe antireflection film to emitted wavelength λ or less. Further, ourexperiments confirmed that, as the surface topology of theantireflection film, the formation of cones or polygonal pyramids(triangular pyramids, rectangular pyramids, hexagonal pyramids, etc.) ata pitch of 0.5 λ or less offered more successful results.

Thus, according to this embodiment, the probability of incident lightsuffering total internal reflections at the interface between the toplayer of the semiconductor multilayer structure including a lightemitting layer and the transparent resin can be reduced by forming anantireflection film whose surface is rough on the light output side ofthe semiconductor multilayer structure. Also, it becomes possible toincrease the light output efficiency significantly by setting thesurface roughness of the antireflection film to the emitted wavelengthor less. In addition, by setting the refractive index of theantireflection film between that of the transparent resin used fordevice packaging and that of the top layer of the semiconductormultilayer structure, the effect of increasing the light outputefficiency can be enhanced further.

Here, in the conventional device, the semiconductor multilayer structurehas a refractive index of about 3.5 and the transparent resin forplastic encapsulation has a refractive index of about 1.5 and hencethere is a large difference in refractive index between them. In thiscase, the critical angle associated with total reflection of lighttraveling from the semiconductor multilayer structure to the transparentresin is small. In this embodiment, the critical angle associated withtotal reflection can be made large by interposing between thesemiconductor multilayer structure and the transparent resin anantireflection film whose refractive index (1.5 to 3.5) is intermediatebetween their refractive indices. Thereby, the light output efficiencycan be increased. Moreover, the light output efficiency can be furtherincreased by making the antireflection film surface rough.

The emitted wavelength of the LED is not restricted to the wavelength ofgreen light. The above effects were also confirmed by products adaptedfor visible light other than green light. Concerning the shape ofprotrusions and recesses (irregularities) of the antireflection filmwhich are of the size of less than the emitted wavelength, we confirmedthat any of surface configurations shown in FIGS. 14A to 14E allowed thelight output efficiency to be increased.

Besides InGaAlP, an InGaAlAs-based material, an AlGaAs-based material ora GaP-based material may be used as the LED material. Further, toprepare the antireflection film, TiO₂, TaO₂ or ZrO₂ may be added toacrylic resin.

[Sixth Embodiment]

FIG. 15 is a sectional view of a green LED according to a sixthembodiment of the present invention.

In this embodiment, the conductivity type of each semiconductor layer ismade opposite to that of the corresponding semiconductor layer in thefifth embodiment. The method of manufacture is substantially the same asin the fifth embodiment. That is, onto a p-type GaAs substrate 120 of250 μm in thickness, a p-type GaAs buffer layer 121 of 0.5 μm inthickness, a p-type In_(0.5)Al_(0.5)P cladding layer 122 of 0.6 μm inthickness, an undoped In_(0.5)(Ga_(0.55)Al_(0.45))_(0.5)P active layer123 of 1.0 μm in thickness, an n-type In_(0.5)Al_(0.5)P cladding layer124 of 1.0 μm in thickness, an n-type InGaP current diffusing layer 125of 1.0 μm in thickness and an n-type GaAs contact layer 126 of 0.1 μm inthickness are grown in the same chamber.

As in the fifth embodiment, an antireflection film 127 having arefractive index of 2.0 is formed on the contact layer 126 by spincoating and then subjected to press shaping using a metal mold so thatits surface becomes roughened. A portion of the antireflection film 127is removed in the place where an electrode is to be formed and an upperelectrode (n-side electrode) 128 is formed on the exposed portion of thecontact layer 126. The GaAs substrate 120 is formed underneath with alower electrode (p-side electrode) 129. The resulting wafer is scribedto obtain chips. Each chip is encapsulated with resin material afterhaving been subjected to a wire bonding step.

Even with such a structure, the light output efficiency was increased bya factor of about 2.5 as in the sixth embodiment. The same effects werealso confirmed by products adapted for visible light other than greenlight. Further, we confirmed that any of the surface configurationsshown in FIGS. 14A to 14E allowed the light output efficiency to beincreased.

[Seventh Embodiment]

In FIG. 8, there is illustrated, in sectional view, the structure of asurface-emitting LD according to a seventh embodiment of the presentinvention.

First, on an n-type GaAs substrate 130 of 250 μm in thickness aresequentially grown an n-type GaAs buffer layer 131 of 0.5 μm inthickness and a multilayer reflecting layer 132 consisting of stackedn-In_(0.5)Al_(0.5)P/n-GaAs films. Subsequently, an n-typeIn_(0.5)Al_(0.5)P cladding layer 133 of 0.6 μm in thickness, an undopedMQW active layer 134 ofIn_(0.5)(Ga_(0.55)Al_(0.45))_(0.5)P/In_(0.5)Ga_(0.5)P, and a p-typeIn_(0.5)Al_(0.5)P cladding layer 135 of 0.6 μm in thickness are grown insequence. Subsequently, a multilayer reflecting layer 136 consisting ofstacked p-In_(0.5)Al_(0.5)P/p-GaAs films, a p-type In_(0.5)Al_(0.5)Pcurrent diffusing layer 137 of 1.0 μm in thickness and a p-type GaAscontact layer 138 of 0.1 μm in thickness are grown in sequence. Theepitaxial layers from the buffer layer 131 to the contact layer 138 aregrown in the same chamber.

Next, a resist mask in the form of a stripe is formed on the contactlayer 138. After that, the layers through the n-type cladding layer 133are then etched away using the resist mask to thereby form a laserridge. Subsequently, an SiO₂ insulating film 141 of 0.5 μm is formedexcept the top of the ridge and then a p-type electrode (Zn-containingAu) is evaporated. Using a resist mask, an upper electrode 142 is thenformed. The upper electrode 142 comes into contact with the peripheralportion of the contact layer 138 and the central portion of the contactlayer is exposed.

Next, an antireflection film 144 having a refractive index of 2.0 andprepared by adding TiO₂ to polyimide resin is formed on the contactlayer 116 by spin coating and its surface is then press shaped by ametal mold having protrusions which are less in size than the wavelengthof emitted light. Thereby, the surface roughness (PV value (max−min)) ofthe antireflection film 117 is made smaller than the wavelength ofemitted light. Afterward, the unnecessary portion of the antireflectionfilm 144 is removed.

Next, the GaAs substrate 130 has its rear side polished to a thicknessof 100 μm and then formed underneath with an n-side electrode(Ge-containing Au) 143. After that, the resulting structure is subjectedto a heat treatment at 450° C. for 15 minutes in an Ar atmosphere.Subsequently, the resulting wafer is scribed to obtain chips. Eachindividual chip is assembled and housed in a package made of epoxy-basedresin (n is about 1.5).

In this embodiment, as in the fifth embodiment, the light outputefficiency can be increased significantly by forming the antireflectionfilm 144 which is intermediate in refractive index between theunderlying semiconductor layers and the sealing resin and has itssurface made rough. Concerning the surface topology of theantireflection film, we confirmed that any of surface configurationsshown in FIGS. 14A to 14E allowed the light output efficiency to beincreased as in the fifth embodiment.

Besides InGaAlP, an InGaAlAs-based material, an AlGaAs-based material ora GaP-based material may be used as the semiconductor material. Further,to prepare the antireflection film, TiO₂, TaO₂ or ZrO₂ may be added toacrylic resin.

[Modifications]

The present invention is not restricted to the embodiments described sofar. Although, in the first and fourth embodiments, the PH₃ partialpressure is set at 10 Pa to make the crystal surface rough, it may liein the range of 1 to 20 Pa. In the third embodiment, to make the crystalsurface rough, annealing is performed with AsH₃ introduced. The gas usedon annealing is not restricted to AsH₃. Any other gas may be usedprovided that it contains hydrogen and a group V element different froma group V element that constitutes the semiconductor layer whose surfaceis to be roughened. The method of making the crystal surface rough isnot restricted to reducing the PH₃ partial pressure and annealing aftercrystal growth. It is also possible to process randomly the surface ofthe semiconductor layer with a grinder having a point angle of less than120 degrees.

The protrusions need not be restricted to circular cones but may bepyramidal ones, such as triangular pyramids, square pyramids, hexagonalpyramids, etc. The protrusions need not necessarily be formed over theentire surface on the light output side. However, it is desired that thepercentage of the area occupied by the protrusions on the light outputsurface be as large as possible. If the percentage is 50% or more, asatisfactory result will be obtained.

The light output efficiency is proportional to the occupied area; thus,if the occupied area by protrusions is 50% or less, the light outputefficiency will be halved (1.1 times or less). If the pitch of theprotrusions is in a range of 0.2 to 0.5 μm, the light output efficiencyis reduced (1.1 times or less). if the pitch is less than 0.2 μm, thegraded index effect will occur.

In the fifth, sixth and eighth embodiments, a metal mold havingprotrusions and recesses is used to make the surface of theantireflection film rough; instead, it is also possible to roughen thesurface of an antireflection film already formed with a grinder. In thiscase, various materials other than resin can be used for theantireflection film.

The requirement that the surface roughness (PV value) be ranged from 200nm to emitted wavelength is not necessarily applied to theantireflection film alone. The requirement may be applied to any otherlayer on the light output side of the semiconductor multilayerstructure. Specifically, the requirement may be applied to the currentdiffusing layer or the contact layer. That is, in the first throughfourth embodiments, the surface roughness (PV value) of the roughenedsurface may be set to emitted wavelength or less. Further, therequirement that the surface roughness (PV value) be the emittedwavelength or less and the requirement that a be 30 degrees or more mayboth be satisfied.

If current can be diffused sufficiently to regions other than just belowthe upper electrode between the upper electrode and the active layer,the current diffusing layer is not necessarily required; it may beomitted. The materials, compositions and thickness of semiconductorlayers forming a light emitting device may be changed according tospecifications.

Although the embodiments have been described taking transparentresin-based encapsulation by way of example, this is not restrictive. Inthe case of no resin-based encapsulation, it is air that comes directlyinto contact with the antireflection film. In this case as well, sincethere is a large difference in refractive index between thesemiconductor multilayer structure and air, the effect of increasing thelight output efficiency resulting from the formation of theantireflection film could be expected likewise.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A surface-emitting semiconductor light emitting device comprising: asubstrate having a major surface; and a semiconductor multilayerstructure formed on the major surface of the substrate and including alight emitting layer, visible light being output from the oppositesurface of the semiconductor multilayer structure and the light outputsurface having been subjected to a roughening process so that a largenumber of protrusions and recesses is formed thereon, said protrusionsand recesses defining peaks and valleys and a height between the maximumpeak and the lowest valley being between 50 nm and a wavelength ofvisible light emitted by the light emitting layer, wherein theprotrusions and recesses are formed periodically with a period set to0.5 λ or less, where λ is the wavelength of the emitted visible light.2. The device according to claim 1, wherein the semiconductor multilayerstructure has a double heterostructure in which an active layer issandwiched between cladding layers, a transparent electrode is formed onthe opposite cladding layer of the double heterostructure from thesubstrate, and a surface of the cladding layer immediately under thetransparent electrode has been subjected to the roughening process. 3.The device according to claim 2, wherein the active layer is made ofInGaAlP and the cladding layers are each made of InAlP.
 4. The deviceaccording to claim 1, wherein the semiconductor multilayer structure hasa double heterostructure in which, an active layer is sandwiched betweencladding layers, a current diffusing layer is formed on the oppositecladding layer of the double heterostructure from the substrate, and thesurface of the current diffusing layer has been subjected to theroughening process.
 5. The device according to claim 1, wherein theprotrusions and recesses are formed at a regular interval and a pitch ofthe protrusions is within a range in which a graded index effect isensured.
 6. The device according to claim 1, wherein the protrusions andrecesses are formed at a regular interval and a pitch of the protrusionsis set to 0.2 μm or less.
 7. A surface-emitting semiconductor lightemitting device comprising: a substrate having a major surface; asemiconductor multilayer structure formed on the major surface of thesubstrate and including a light emitting layer, visible light beingoutput from the opposite surface of the multilayer structure from thesubstrate; and an antireflection film formed on the light output surfaceof the semiconductor multilayer structure and having its surfaceroughened so that a large number of protrusions and recesses is formedthereon, said protrusions and recesses defining peaks and valleys and aheight between the maximum peak and the lowest valley being between 50nm and a wavelength of visible light emitted by the light emittinglayer, wherein the protrusions and recesses are formed periodically witha period set to 0.5 λ or less, where λ is the wavelength of the emittedvisible light.
 8. The device according to claim 7, wherein a refractiveindex of the antireflection film is set higher than that of atransparent resin which is applied to the light output surface of thesemiconductor multilayer structure but lower than that of a top layer ofthe semiconductor multilayer structure.
 9. The device according to claim7, wherein the semiconductor multilayer structure has a doubleheterostructure in which an active layer is sandwiched between claddinglayers and a current diffusing layer is formed on the opposite claddinglayer of the double heterostructure from the substrate.
 10. The deviceaccording to claim 9, wherein the active layer is made of InGaAlP andthe cladding layers are each made of InAlP.
 11. The device according toclaim 7, wherein the protrusions and recesses are formed at a regularinterval and a pitch of the protrusions is within a range in which agraded index effect is ensured.
 12. The device according to claim 7,wherein the protrusions and recesses are formed at a regular intervaland a pitch of the protrusions is set to 0.2 μm or less.
 13. Asurface-emitting semiconductor light emitting device comprising: asubstrate having a major surface; a semiconductor multilayer structureformed on the major surface of the substrate and including a lightemitting layer, visible light being output from the opposite surface ofthe multilayer structure from the substrate; a first electrode formed inselected areas of the light output surface of the semiconductormultilayer structure; an antireflection film formed on the light outputsurface of the semiconductor multilayer structure except the areas ofthe first electrode and has its surface roughened so that a large numberof protrusions and recesses is formed thereon; a second electrode formedover the entire rear surface of the substrate; and said protrusions andrecesses defining peaks and valleys, and a height between the maximumpeak and the lowest valley being between 50 nm and a wavelength ofvisible light emitted by the light emitting layer, wherein theprotrusions and recesses are formed periodically with a period set to0.5 λ or less, where λ is the wavelength of the emitted visible light.14. The device according to claim 13, wherein a refractive index of theantireflection film is set higher than that of a transparent resin whichis applied to the light output surface of the semiconductor multilayerstructure but lower than that of a top layer of the semiconductormultilayer structure.
 15. The device according to claim, 13, wherein thesemiconductor multilayer structure has a double heterostructure in whichan active layer is sandwiched between cladding layers and a currentdiffusing layer is formed on the opposite cladding layer of the doubleheterostructure from the substrate.
 16. The device according to claim15, wherein the active layer is made of InGaAlP and the cladding layersare each made of InAlP.
 17. The device according to claim 13, whereinthe protrusions and recesses are formed at a regular interval and apitch of the protrusions is within a range in which a graded indexeffect is ensured.
 18. The device according to claim 13, wherein theprotrusions and recesses are formed at a regular interval and a pitch ofthe protrusions is set to 0.2 μm or less.
 19. A surface-emittingsemiconductor light emitting device comprising: a substrate made of acompound semiconductor of a first conductivity type; a doubleheterostructure formed on the substrate and comprising a cladding layerof the first conductivity type, an active layer, and a cladding layer ofa second conductivity type; a current diffusing layer of the secondconductivity type formed on the cladding layer of the secondconductivity type.of the double heterostructure; a contact layer of thesecond conductivity type formed on the current diffusing layer; an upperelectrode selectively formed on the contact layer; a lower electrodeformed on the rear surface of the substrate; and an antireflection filmformed on the contact layer except its portions where the upperelectrode is formed, the antireflection film having its surfaceroughened so that a large number of neighboring protrusions and recessesis formed thereon, said protrusions and recesses defining peaks andvalleys, and a height between the maximum peak and the lowest valleybeing between 50 nm and a wavelength of visible light emitted by thelight emitting layer, and the protrusions and recesses are formedperiodically with a period set to 0.5 λ or less, where λ is thewavelength of the emitted visible.
 20. The device according to claim 19,wherein the protrusions and recesses are formed at a regular intervaland a pitch of the protrusions is within a range in which a graded indexeffect is ensured.
 21. The device according to claim the protrusions andrecesses are formed at a regular interval and a pitch of the protrusionsis set to 0.2 μm or less.